Sound waves are the primary tool used to search for oil and gas reserves beneath the Earth's strata. Sound waves are convenient because they can propagate over long distances and penetrate into complex layered media to obtain important information regarding the presence, composition, and physical extent of reserves. This is the case for surveys conducted on both land and water. Although a variety of methods have been used to generate sound waves in water, the primary technique over the past three decades is the use of air guns, which expel short bursts of high-pressure air. The creation and collapse of air bubbles produced by this process causes high-energy sound waves to be directed toward the seafloor with approximately 98% of the energy generated over the frequency range from 5 to 200 Hz. The waves penetrate into the strata and differentially reflect back towards the surface where they are recorded by an array of receivers (i.e., hydrophones).
Generally speaking, marine seismic surveys are performed by towing 12 to 48 air guns 300 to 500 m behind a survey vessel at depths on the order of 1 to 10 m. The air guns are typically configured in a planar array and produce source levels up to 240 dB are 1 μPa-m. These are impulsive sounds that result from the sudden discharge of high-pressure air and have a duty cycle ranging from 5 to 20 seconds. The receiving array that records the direct and reflected sound waves is usually comprised of up to 16 streamers each with hundreds of hydrophones. The streamers are typically 3 to 12 km long and have a lateral spacing anywhere from 50 to 100 m. The source-receiver configuration coupled with tow speeds ranging from 1.5 to 2.5 m/s facilitates survey rates on the order of 10 km2/day.
For more information on marine seismic surveys, please consult “Marine Geophysical Operations: An Overview,” International Association of Geophysical Contractors (June 2009), or “An Overview of Marine Seismic Operations,” International Association of Oil and Gas Producers, Report No. 448 (April 2011), each incorporated by reference herein. For more information on air guns used in marine seismic surveys, please see, e.g., W. Dragoset, “An Introduction to Air Guns and Air-Gun Arrays,” The Leading Edge, 19 (8), 892-897 (2000) or J. Caldwell and W. Dragoset, “A Brief Overview of Seismic Air-gun Arrays,” The Leading Edge, 19 (8), 898-902 (2000), each incorporated by reference herein.
For many years there have been growing concerns by environmentalists, scientists, and governments that increasing sound levels in the marine environment might be detrimental to a variety of marine life. Marine mammals are the primary concern along with fish and invertebrates which are secondary. A review of these issues as they pertain to marine mammals can be found in “Airgun Arrays and Marine Mammals,” International Association of Geophysical Contractors (August 2002), incorporated by reference herein. Because of the increasing concerns over potential sound effects to marine biota, there are growing concerns that marine seismic surveys could be significantly restricted by future regulations. With these concerns in mind, the oil and gas industry has considered alternatives to air guns, and in particular using marine vibrators that can provide a coherent (i.e., non-impulsive) source of acoustic energy to enhance the efficacy of the system. There are numerous features that marine vibrator-based seismic survey systems offer that are important as they relate to environmental considerations. For example:                (1) The use of a coherent source can provide excitation over a much longer time interval than an incoherent (i.e., impulsive) source, such as an air gun, which is thought to pose lower risk to marine life because the same nominal energy, over a specific band of interest, can be spread-out over time rather than being concentrated at an instant;        (2) The use of a coherent source has a greater propensity to reduce or eliminate high-frequency components (e.g., sounds greater than 100 Hz) relative to an incoherent source since the frequency domain representation of the signals associated with a coherent source is typically concentrated at deterministic and controllable frequencies thereby confining the sound energy to a specific band which can be tailored to provide minimal risk to certain species of marine life.        
A comprehensive review of the environmental impact of marine seismic surveys performed using marine vibrators as opposed to air guns can be obtained from “Environmental Assessment of Marine Vibroseis,” prepared by LGL, Ltd and Marine Acoustics, Inc., LGL Report TA4604-1, JIP Contract 22 07-12 (April 2011), incorporated by reference herein.
While there are several environmental advantages associated with the use of coherent sound sources for marine seismic surveys, there are also benefits of using them to improve the performance of the survey system as a whole. For example, the use of coherent sound sources for marine seismic surveys allows the excitation signal associated with the projector array and the signal processing algorithms associated with the receive array to be tailored so that the most accurate image of the layered media under evaluation is created. Some examples of excitation signals include, but are not limited to frequency-modulated (FM) sweeps and pseudo-random noise (PRN).
Resident in the characteristics of these signals is the ability of the projector (or multiple projectors used in an array) to be controlled in a precise manner, which is not an easy or straightforward proposition with incoherent sources like air guns. Data collection systems that rely on FM and PRN signals can employ signal processing techniques such as matched filters to improve the signal-to-noise ratio without the need for increasing the source level. Processing gains can also be achieved through long integration times associated with coherent signals which are generated for extended periods of time. Reference textbooks that provide the details of these processing techniques as well as others include W. S. Burdic, Underwater Acoustic System Analysis, Prentice Hall, Inc. (1984); and A. D. Whalen Detection of Signals in Noise, Academic Press (1971), each incorporated by reference herein.
Historically speaking, the use of coherent sound sources in connection with marine seismic surveys has not been widespread, but some devices have been reduced to practice. Marine vibrators, comprised of a hydraulically actuated piston and a flextensional transducer, have been developed as described in W. D. Weber and G. R. Johnson, “An Environmentally Sound Geophysical Source—The Transition Zone Marine Vibrator,” Conference Proceedings, Society of Petroleum Engineers, Paper SPE 46805 (1988) and “PGS Electrical Marine Vibrator,” TechLink—A Publication of PGS Geophysical, 11 (5) (November 2005), each incorporated by reference herein. The concept of a hydraulically actuated piston designed for in-water use is an adaptation of the principal transduction mechanism employed by land vibrators such as those developed by Industrial Vehicles International (Tulsa, Okla.). The flextensional transducer generally conforms to a Class IV design and utilizes electrical coils in connection with a magnetic circuit as the principal transduction method. This was done in contrast to the traditional use of a longitudinal piezoelectric actuator so that the device could meet performance requirements in the operational band of interest (i.e., nominally 5 to 100 Hz). Interestingly, two separate flextensional transducers are required to cover the entire band.
As discussed further below, aspects of the present invention rely on a piston-type sound projector that employs an electro-magnetic driver which conforms to a moving armature force generator. See, for example, U.S. Pat. Nos. 5,206,839, 5,266,854, and 5,587,615. U.S. Pat. No. 5,206,839 describes a device configured as an underwater sound projector. U.S. Pat. Nos. 5,266,854 and 5,587,615 describe devices configured as vibratory shakers. The devices described in U.S. Pat. Nos. 5,206,839 and 5,266,854 utilize a magnetic circuit comprised of E-shaped and I-shaped laminated structures wherein the E-shaped structure is fitted with separate windings for alternating and direct currents (AC and DC). The DC signal provides a magnetic bias that is used in concert with an appropriately phased AC signal to generate motion in the I-shaped structure which constitutes the moving armature assembly. This arrangement results in force output that is linear with the applied current. It is noted that without the DC magnetic bias, moving armature transducers are inherently nonlinear and tend to be undesirable for use in numerous applications.
The device described in U.S. Pat. No. 5,587,615 foregoes the use of a DC winding in favor of a permanent magnet, resulting in a magnetic circuit design that employs multiple elongate laminated structures (i.e., magnetic cores) that contain AC windings and a single permanent magnet positioned between the cores to provide the magnetic bias. This arrangement is substantially different than that described in U.S. Pat. Nos. 5,206,839 and 5,266,854, but provides force output in the moving armature assembly that is linear with the applied current.